The present invention relates to a joint driving device applicable to a joint mechanism for a mechanical apparatus such as a robot arm, and also relates to a robot arm including the joint driving device.
Elastic body actuators such as pneumatic artificial muscles can generate translation displacements, but can generate forces only in the direction of their contraction. Accordingly, when such an elastic body actuator is applied to a rotational joint mechanism for a movable mechanical apparatus such as a robot arm, an antagonistic driving mechanism as illustrated in FIGS. 14A and 14B is employed to realize rotational motions.
In FIGS. 14A and 14B, a numeral 57 denotes a first configuration member and a numeral 58 denotes a second configuration member. The first configuration member 57 and the second configuration member 58 are coupled to each other through a rotational joint 59 and, therefore, they can be relatively rotated with respect to each other. Actuator supporting members 60a and 60b are placed on the first configuration member 57 such that they face each other, while an actuator driving-force transfer member 61 is placed on the second configuration member 58.
Numerals 62a and 62b denote elastic body actuators which are placed such that they are in parallel with the first configuration member 57 and sandwich the first configuration member 57. The elastic body actuators 62a and 62b are coupled to the actuator supporting members 60a and 60b, respectively, through rotation joints 63a and 63b. Further, the elastic body actuators 62a and 62b are coupled to the actuator driving-force transfer member 61 through rotation joints 63c and 63d, respectively, which enables rotational motion about the rotational joint 59 with the contraction of the elastic body actuators 62a and 62b. 
With the antagonistic driving mechanism of FIGS. 14A and 14B, when the rotation shaft is rotated, as in the cases of FIGS. 14A and 14B, for example, the angle α between the elastic body actuator 62a and the actuator driving-force transfer member 61 as a coupling supporting member is different between when the rotational angle θ is smaller and when the rotational angle θ is greater. Accordingly, even when the elastic body actuators generate a certain translational force, the rotational torque applied to the rotation shaft is varied depending on the rotational angle θ. Further, even when the elastic body actuators generate a certain translation displacement, the rotational displacement of the rotational shaft is varied depending on the rotational angle θ. Specifically, when the rotational angle θ is greater, a smaller rotational torque is generated and a smaller rotational displacement is generated, in comparison with cases where the rotational angle θ is smaller.
Further, as the portion designated by an arrow as a symbol A in FIG. 14B, when the rotational angle θ is greater, the interval between the elastic body actuators is reduced, which causes the end portion of the elastic body actuator 62a or 62b to come into contact with the first configuration member 57, thereby causing the issue of reduction of the movable range of the rotational angle θ.
To cope with the aforementioned issue, in the prior art, Patent Document 1 (Japanese Unexamined Patent Publication No. 07-24772) provides a wire/pulley driving configuration capable of generating a constant rotational torque regardless of the rotational angle θ with a certain translational force generated from elastic body actuators and also capable of generating a constant rotational displacement regardless of the rotational angle θ with a certain translation displacement of the elastic body actuators.
Further, with the configuration of Patent Document 1, it is possible to place an intermediate pulley for placing the elastic body actuators in parallel with each other even if the pulley diameter is smaller, which prevents the elastic body actuators from coming into contact with each other, thereby preventing the movable range from being restricted.
However, the aforementioned configuration of Patent Document 1 may induce looseness of wires and fall from the pulleys, which may reduce the control accuracy or may make it impossible to perform operations.
Further, the aforementioned configuration is capable of generating only a certain rotational torque with a certain translational force generated from the elastic body actuators. Therefore, when the aforementioned configuration is applied to a robot and the like, the configuration is not capable of generating variable torques with a certain translational force depending on the situation, such as situations which require a greater force for griping an object with a greater weight and situations which require a greater operable range for operating the robot over a wider range.
It is an object of the present invention to provide a joint driving device and to provide a robot arm including the aforementioned joint driving device, with higher reliability and higher flexibility which can overcome the aforementioned issues of conventional joint mechanisms, can eliminate the issues of looseness of wires and fall from pulleys and the like, and can generate variable torques depending on the situation with a certain translational force generated from elastic body actuators.